Apparatus for detecting the orientation of the plane of polarization of a plane-polarized beam of radiation

ABSTRACT

An apparatus is described for detecting the orientation of the plane of polarization of a plane-polarized beam of radiation by means of a radiation-sensitive detection system. It is shown that the insertion of an isotropic beam-splitting element in the path of the beam and the insertion of a series arrangement of at least three birefringent elements at least one of which is an electro-optical crystal in each of the paths of the sub-beams and the application to the electro-optical crystals traversed by one of the sub-beams of periodic voltages which differ in phase by one-fourth cycle with the periodic voltages applied to the electro-optical crystals traversed by the other sub-beam enable the orientation of a rotating plane of polarization to be accurately determined.

United States Maria Mes [72] Inventor: Johannes Antonius Maria Mes, Em-

masingel, Netherlands [73] Assignee: U.S. Philips Corporation, New

York, NY.

[22] Filed: Feb. 4, 1971 [211 Appl. No.: 112,721

[30] Foreign Application Priority Data Feb. 7, 1970 Netherlands..7001772 [52] US. Cl. ..350/l50, 350/157, 350/160 [51] Int. Cl ..G02f1/26 [58] Field of Search ..350/147, 150, 157,160-161 [56] ReferencesCited UNITED STATES PATENTS 3,366,792 l/1968 Ohm ..350/150 151 8,685,8831 Aug. 22,1972

H1971 DeLang et al. ..350/l57 Primary Examiner-David Schonberg AssistantExaminer-Paul R. Miller Attorney-Frank R. Trifan' It is shown that theinsertion of an isotropic beamsplitting element in the path of the beamand the insertion of a series arrangement of at least three birefringentelements at least one of which is an electro-optical crystal in each ofthe paths of the subbeams and the application to the electro-opticalcrystals traversed by one of the sub-beams of periodic voltages whichdiffer in phase by one-fourth cycle with the periodic voltages appliedto the electro-optical crystals traversed by the other sub-beam enablethe orientation of a rotating plane of polarization to be accuratelydetermined.

3 Claim 3 Drawing Figures PAIENTEnAucza m2 INVEXTOR.

JOHANNES AM. MES

BY ,2 AGZNT APPARATUS FOR DETECTING THE ORIENTATION OF THE PLANE OFPOLARIZATION OF A PLANE-POLARIZED BEAM OF RADIATION The inventionrelates to an apparatus for detecting the orientation of the plane ofpolarization of a planepolarized beam of radiation by means of aradiationsensitive detection system.

The invention relates also to an apparatus which enal traversed by oneof the sub-beams periodic voltages which differ in phase by one-fourthcycle with the periodic voltages applied to the correspondingelectrooptical crystals traversed by the other sub-beam.

It should be noted that a series arrangement of three birefringentelements at least one of which is an electro-optical crystal is known.

The desired phase shift is realized by electro-optical means, anelectronic phase shift being dispensed with.

The series arrangement preferably comprises two N4 plates having theiroptic axes oriented in the same direction between which there has beenplaced an electro-optical crystal having an orientation which differs by45 percent from that of the M4 plates, a suitable periodicelectricvoltage being applied to the crystal.

The crystal may comprise several component crystals so that theamplitude of the voltage applied to each component crystal can besmaller than that applied to the undivided crystal by a factorproportional to the number of component crystals.

An embodiment of the invention will now be described, by way of example,with reference to the accompanying diagrammatic drawings, in which:

FIG. 1 shows a known apparatus for dynamically detecting the orientationof the plane of polarization of a beam of radiation;

FIG. 2 shows in block-schematic form the associated electronic devicefor processing the signals, and

FIG. 3 shows an apparatus according to the invention.

In the apparatus shown in FIG. 1, plane-polarized radiation whichemanates from a source 1 and is converted into a parallel beam by a lens2 falls on the series arrangement of a N4 plate 4, a KDP-crystal 5 and aM4-plate 6, in that order. The optic axes 7 and 9 of the M4 plates 4 and6 are parallel to one another, and the optic axes 8 of the crystal 5 isinclined at an angle of 45 to that of the plates 4 and 6. An alternatingvoltage V= V,, sin I is applied to the crystal 5. The emergent beam ofradiation is divided in two sub-beams by a beamsplitting mirror and thesub-beams fall on polarizers 21 and 22 the direction of polarization 23and 24 respectively of which are at an angle of 45 to one another. Thebeams which emerge from the polarizers 22 and 21 are converted bydetectors 33 and 34 respectively into electrical voltages of the form:

S, ==constant +S sin (kz b sinwt) and S constant S cos (kz b sinwt),

where kz indicates the orientation of the plane of polarization of theradiation emitted by the source 1.

These voltages may be processed in the manner which will now bedescribed with reference to FIG. 2.

Band-pas filters 35 and 36 transmit only the com- 0 ponents:

S',=A cos (kz) sin an and S',=A sin (kz) sin wt of the signals S, and Srespectively. Further processing requires a phase shift of one of thesignals with respect to wt. However, for considerations of symmetry bothsignals are shifted in phase. A phase shifter 37 shifts S through +45and a phase shifter 38 shifts S, through 45. This corresponds to a phaseshift of S, relative to 8' through +90. The signals V, and V' then willhave the form:

V',=A coskz cos wt and V' =Asinkzsinwt (I) and V, are applied toamplifiers 39 and 40, in which they are added to one another andsubtracted from one another respectively, resulting in the followingsignals:

These signals can be processed in known manner to produce digitalcounting pulses. 7

However, only for a given fixed frequency can the electronic phase shiftbe effected with the accuracy required for a correct determination ofthe orientation of the plane of polarization. The signals S, and S',,which may be written:

have two frequencies which are constant and equal only when in otherwords when the plane of polarization of the radiation emitted by thesource 1 is stationary. When this plane of polarization is notstationary, the phase shift and hence the indication of the orientationgiven by the above-described apparatus will become less accurate.

The apparatus according to the invention enables the orientation of theplane of polarization to be determined with greater accuracy even whenthe plane of polarization is rotating. Tee invention is based on therecognition of effecting the phase shift by electro-optical means sothat the electronic phase shifters may be dispensed with and thedisadvantages inherent in them are avoided.

FIG. 3 shows an embodiment of an apparatus according to the invention.

The plane-polarized radiation emitted by the source 1 and converted intoa parallel beam by a lens 2 is divided into two sub-beams by anisotropic beamsplitting mirror 20. In the path of each sub-beam there isinserted a modulating device 41 and 42 respectively which comprises anelectro-optical crystal and two M4 plates. The devices 41 and 42 eachcomprise the elements 4, 5 and 6 of FIG. 1. After having been modulated,the sub-beams pass through polarizers 22 and 21 respectively. Thedirections of polarization 23 and 24 of the polarizers 21 and 22respectively are inclined at an angle of 45 to one another. The beams ofradiation which emerge from the polarizers 21 and 22 are intercepted andconverted into electric signals by detectors 34 and 33, respectively.

The electro-optical crystal of the modulating devices 41 and 42 arecontrolled by voltages:

V %in wt and V =Bcos wt respectively. The output signals of thedetectors 33 and 34 will then be:

S =constant +S sin (kz +b sin (at) and S =constant- S cos (kz +b cos a)respectively. From these signals there are produced by filtering in afrequency band about the center frequency w the signals:

S',=Acoskzsinwtand S' =A sinkzcoswt S,,=A sin (kz+wl) and S =A sin whichmay again be processed in known manner to form digital counting pulses.The said processing operations may again be perfonned by means of adevice as shown in FIG. 2, which, however, does not include the phaseshifters 37 and 38.

It should be noted that apart from the absorption in the electro-opticalcrystals the intensities of the beams of radiation incident on thedetectors are equal in'the apparatuses shown in FIGS. 1 and 3.

In the above described apparatuses two sinusoidal signals are applied tothe electro-optical signals. However, alternatively two periodic signalsof another form, such as periodic triangular signals, may be applied tothe electro-optical crystals, provided that these signals have a mutualphase difference of one-fourth cycle.

YKir'ifififiibr detecting the orientation of the plane of polarizationof a plane-polarized beam of radiation by means of a radiation-sensitivedetectionsystem, an isotropic beam-splitting element in the path of thebeam, a series arrangement of at least three birefringent elementspositioned in the path of each sub-beam produced by the beam splittingelement, at least one of the birefringent elements being an electroopticcrystal, a separate polarizer in the path of radiation eminating fromeach series arrangement, and means for applying to the electro-opticalcrystals traversed by one of the sub-beams periodic voltages whichdifi'er in phase by one-fourth cycle with the periodic voltages appliedto the corresponding electrooptical crystals traversed by the othersub-beam.

2. An apparatus as claimed in claim 1, wherein the series arrangementcomprises two M4 plates which have optic axes oriented in the samedirection and between which there is arranged an electro-optical crystalhaving an optic axis which difl'ers by 45 from that of the M4 plates, asuitable periodic electric voltage being applied to this crystal.

3. An apparatus as claimed in claim 2, said electrooptical crystalcomprising n component crystals, the amplitude of the periodic voltageapplied to each component crystal being l/n times the periodic voltagenecessary for exciting a crystal in a single crystal elec- .tro-opticmodulator.

UNITED STATES PATENT OFFICE (s/ee) CERTIFICATE OF CORRECTION Patent N3,685,883 Dated August 22, 1972 Inventor) JOHANNES ANTONIUS MARIA MES Itis certified that error appears in the above-identified patent and thatsaid Letters Patent are hereby corrected as shown below:

Col. 1, line 30, "their optic a xes orientedin the same" should be--their principal directions oriented in the same direction-;

line 31, cancel "direction" line 56, "optic axes" should'be --principaldirections-; line 58, "optic a es" should be principal directions-;

line 60, "Qft" should be UJt-;

Col. 2, line 67, "Tee" should be -The IN THE CLAIMS Claim 2, line 3,"optic axes oriented in the same direction" should be -principaldirections oriented in the same direction-;

line 5, "an optic axis" should be -a principal direction-.

signed and sealed this- 9th day of January 1973 (SEAL) Attest:

EDWARD M.FLETCHER,JR. ROBERT GOTTSCHALK Attesting Officer e Commissionerof Patents

1. An apparatus for detecting the orientation of the plane of polarization of a plane-polarized beam of radiation by means of a radiation-sensitive detection system, an isotropic beam-splitting element in the path of the beam, a series arrangement of at least three birefringent elements positioned in the path of each subbeam produced by the beam splitting element, at least one of the birefringent elements being an electro-optic crystal, a separate polarizer in the path of radiation eminating from each series arrangement, and means for applying to the electro-optical crystals traversed by one of the sub-beams periodic voltages which differ in phase by one-fourth cycle with the periodic voltages applied to the corresponding electro-optical crystals traversed by the other sub-beam.
 2. An apparatus as claimed in claim 1, wherein the series arrangement comprises two lambda /4 plates which have principal directions oriented in the same direction and between which there is arranged an electro-optical crystal having a principal direction which differs by 45* from that of the lambda /4 plates, a suitable periodic electric voltage being applied to this crystal.
 3. An apparatus as claimed in claim 2, said electro-optical crystal comprising n component crystals, the amplitude of the periodic voltage applied to each component crystal being 1/n times the periodic voltage necessary for exciting a crystal in a single crystaL electro-optic modulator. 